Can't get hot water heater to work - Rinnai Infinity 18

Hi there - we just moved into a rental property and have no hot water. We have a Rinnai Infinity 18 mounted on the wall outside the 1st floor (quite hard to access). When we turn on the hot water tap, we hear the pilot trying to light, and a fan going, and we have turned the gas inlet valve and the water valve to the on/open position (both at the meter, and locally at the Rinnai). The control panel in the bathroom has the temperature we want on it - but after running the water for a bit, it changes to the cold temperature (11 or 12) and flashes. Yesterday the 'in use' light illuminated, though it still didn't work - today, it doesn't even illuminate! The unit seems to have power, water and gas... but we still don't have hot water. The gas cooktop inside the house is working, and the lights and water are, too.
What else can we try? I'm sure the last renters here had hot water, so I'm certain what silly thing we're missing?? Thanks very much for your help!

If you can get to the gas valve make sure it is in the on position and not in pilot or off. Does it sound like
a spark ignition system? Even though you say the gas valve is on, by code thered should be a gas shutoff valve located within site of the unit. Follow the
gas pipe until you find such a valve and make sure its on

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If you aren't getting any error codes, then the heater is probably doing its job. Your problem is probably somewhere in your plumbing system. The most common reason you get this sort of problem is due to cold water mixing in the hot water system. There are a number of causes for a "cross connection" :

The easiest way to confirm a cross connection is to: 1) shutoff the cold water entering the tankless heater 2) open hot water valves around the house. If you still have water flowing at your hot water fixtures, then you have a cross connection.

We have the same problem! It's happened twice this week after heavy rain. There is no error code "19"listed in the manual. Ours is a Rinnai Infinity 24 hot water service. I called Rinnai and they had no idea and were going to send a serviceman when it suddenly righted itself. Now it's recurred. Wife and daughters furious. Please help!

Here are two links to get the manual for your unit. Since I don't have the model number for your unit, you can find this in one of the two links. If you have the manual, you will know if you are skipping a step or not. If you have done everything, then I would call Rinnai to find an authorized tech for your appliance. Hope this helps.

sounds like the water line is frozen before it gets to heater. You can take a blow dryer and try to thaw the line.You should be able to hook up a hose on the hot side of water heater. turn valve off so it wont flow into house.You can turn on hot water valve on in kitchen but you will have to thaw cold and hot line. Make sure you know where water shut off is to the house just in case a line breaks.

We had the same problem - the hot water would only be hot for 3 -5 minutes and shut off, then we had to turn the water off, wait a few minutes, and do it again. VERY FRUSTRATING.

Solution: The Rinnai serviceman showed me the heat-exchanger blower/fan: It was clogged with dust and moths.

Another clue: Our Rinnai was extremely noisy when the heat came on. After the fan was cleaned and replaced, it is nearly silent.

If you are reasonably machanically inclined:
TURN THE BREAKER / POWER OFF to the unit.

The blower-fan is located in the lower central area of the unit (at least on our unit). If you draw imaginary north-south, east-west lines through the center of the unit, the fan is in the lower-left quadrant. There is a spring clamp securing the black rubber accordian-like tube to the fan. There are two screws that secure the fan to the unit. there are two plugs that connect to the fan, as well as a third air-flow sensor that should be removed. then you can look at the fan unit, as well as the grill on the underside of the heat-exchanger that should be cleaned.

My thought was to go out and get a simple bug-screen, and place it over the air-input outlet vent going outside, but the repair guy said that although it may sound good, if that screen ever gets clogged, or reduces airflow to the Rinnai, then the unit will work harder, or even shut off, giving the same problem,